What to put under my tiles

Hardie board .

well first a way use bigger board , make sure there is no moovement, use every 150mm space between screws. And the last and VERY important use Ditra mattings, and designed adhesive for it. If you do under floor heating must have right adhesive for it as well, normally it would sais on bag. Hope it helps

If you have a timber floor and want to keep the height the same then a 6mm cement board would be the best solution. There are a number of manufacturers that produce these, the main ones being Hardie or No More Ply. These boards need to be glued using the recommended glue and then screwed down making sure you stagger the joints. Make sure you use the recommended glue and screws. If the timber floor has any movement then it is always advisable to screw these solid before laying the new boards, essentially you are trying to eliminate any movement/bounce in the substrate. The cement board, if laid correctly also provides you with a fully waterproof floor and No More Ply sell a pre primed board for time saving too. If you follow the manufacturers installation recommendations this will also provide you with a lengthy guarantee on the product.

Cement boards 6mm they don't dwell if damp or wet

You can use 6mm Hardie Backer board or my choice would be a product called Delta board. That also comes in 6mm thickness & has a thin layer of polystyrene running through the middle to give extra sound proofing, flexibility, waterproofing & insulation too. It should be screwed down every 6inches & can go straight onto floorboards or ply. It’s easy to cut & install. Ditra matting is only really for lateral movement not up & down movement. As long as the boards are secured right to minimise any bounce along with using an S2 highly flexible adhesive & grout you shouldn’t have any problems.